Telomerase reverse transcribes telomere repeats onto chromosome termini. The long- term goal of this project is to develop the nematode C. elegans as a system for the study of telomerase and telomere biology. C. elegans possesses holocentric chromosomes that facilitate genetic isolation of initial end-to-end chromosome fusion events that occur when telomerase is deficient. Isolation and analysis of end-to-end fusions from various genetic backgrounds will provide unprecedented insight into their genesis and may reveal unexpected functions of some DNA damage response proteins that interact with normal telomeres. Genetic screens have identified four proteins that are required for telomerase to function at telomeres in yeast, whereas our forward genetic screens indicate that 10 or more proteins are required in C. elegans and perhaps in other multicellular organisms. One model in the field suggests that telomeres may be sensed as DNA double-strand breaks prior to recruitment of telomerase. However, we have determined that the MRT-1 nuclease, which facilitates DNA interstrand crosslink repair, is required for telomerase to act at C. elegans telomeres. Thus, telomerase may be recruited to telomeres via a pathway that can respond to DNA interstrand crosslinks. We suspect that this hypothesis may be relevant to telomere evolution and plan to test this possibility using genetic and biochemical approaches. In addition, we shall identify and characterize four new mutations that result in deficiency for telomerase activity in vivo in C. elegans (279, 222g, 36f and 3211e). Finally, further mutant screens will be performed in an effort to saturate for genes required for telomerase activity in vivo. Telomerase only acts at telomeres for a fleeting moment during the cell cycle, so the powerful genetics of C. elegans may help to identify the suite of genes responsible for telomerase activity in vivo in multicellular organisms. PUBLIC HEALTH RELEVANCE Deficiency for telomerase causes lethal human hereditary disorders such as aplastic anemia, dyskeritosis congenita and pulmonary fibrosis: genes that mutate to cause these human diseases may be defined by this research project. Given that most cancers express telomerase whereas most normal somatic cells do not, our studies may also help to define proteins and/or pathways that may be useful targets for treatment of a wide variety of cancers. Finally, this project may define recombination intermediates and pathways that are relevant to telomere-induced genome instability, which may promote tumor development.